1. Field of the Invention
This invention relates to a method of recovering chlorine gas, and in particular, to a method of recovering chlorine gas from calcium chloride.
2. Description of the Related Art
Since the mass production of plastic leads to the increased consumption and increased waste of plastic, it is feared that the disposal of waste plastic may badly affect the environment. In particular, since polyvinyl chloride contains a large quantity of chlorine, the environmental safety thereof now becomes a serious problem. Recently, new regulations called “Regulations On polychlorinated-dibenzo-p-dioxins (PCDDs)” have been enforced wherein it is stipulated, for the purpose of suppressing the generation of dioxin, to set the temperature of a garbage incinerator to not less than 800° C. and to perform the incineration of polyvinyl chloride in the presence of a sufficient quantity of oxygen gas and under the condition wherein the residence time of decomposed gases is set to not less than three seconds. However, even if the generation of organic chlorine compounds such as dioxin can be suppressed by this new regulation, the discharge of inorganic chlorine compounds (hydrogen chloride) would be increased in proportion to the reduction of the organic chlorine compounds. Therefore, it is now desired to develop a technique for safely and economically processing hydrogen chloride.
Meanwhile, even in a chemical plant where phosgene is employed as a raw material, a large quantity of hydrogen chloride is generated. For example, in the synthesizing process of methylene diisocyanate (MDI) or toluene diisocyanate (TDI), which are useful as raw materials for the production of polyurethane as well as in the synthesizing process of polycarbonate (PC) from bisphenol, a large quantity of hydrogen chloride is generated as phosgene is employed as a raw material in these synthesizing processes. Under the circumstances, attempts are now being made to recover chlorine gas from hydrogen chloride generated in this manner so as to recycle chlorine for the production of phosgene. For example, it is reported that it is possible, through the reaction of hydrogen chloride with oxygen in the presence of a chromium oxide catalyst supported by a silica, to recover 70% of hydrogen chloride as chlorine gas (U.S. Pat. Nos. 4,822,589 and 4,803,065). Further, European Patent No. 936184A2 describes that it is possible, through the employment of a ruthenium oxide catalyst supported by a titania, to recover 90% of hydrogen chloride as chlorine gas. Even in the United States of America, a consortium is organized with Air Products Co. being a leading member to engage in the recovery of chlorine gas from hydrogen chloride. The method now being studied in the United States of America is called the Catalyst Carrier Process (CCP) method, wherein a pair of reaction columns are employed to recover chlorine gas from hydrogen chloride. In this case, the first reaction column is employed for generating copper chloride through the reaction between hydrogen chloride and copper oxide. The resultant copper chloride is then transferred, by a riser, to the second reaction column wherein the copper chloride is allowed to react with oxygen gas blown therein from a lower portion of the second reaction column to produce copper oxide concurrent with the production of chlorine gas, thus recovering chlorine gas (U.S. Pat. Nos. 4,994,256 and 5,154,911).
The above-mentioned chlorine gas-recovery methods however are all directed to high-purity hydrogen chloride (99% or more in purity) which will be by-produced from a chemical plant. Since the concentration of hydrogen chloride in the exhaust gas from an incinerator is about 1% at most, these conventional chlorine gas-recovery methods cannot be used for the recovery of chlorine gas from the exhaust gas of an incinerator.
Currently, the exhaust gas from an incinerator is sprayed with a slurry-like aqueous solution of slaked lime (Ca(OH)2) or with quicklime (CaO) fine powder to immobilize chlorine as calcium chloride (CaCl2), which is then buried in the ground, thus disposing of the calcium chloride. This calcium chloride itself is designated as a specially controlled waste, so that the cost for burying calcium chloride is relatively high, and moreover, this disposed method is not necessarily safe.
If a suitable method of recovering chlorine from calcium chloride can be found, it is no longer necessary to bury calcium chloride in the ground, and at the same time, the chlorine that is recovered can be effectively recycled as a resource.